Study On Hydrogen Storage And Electrochemical Performances Of C14 Laves Phase And BCC Phase Alloys

Ti-V-based BCC phase alloys have attractive prospect for use as hydrogen storage media for fuel cell and negative active materials for Ni-MH batteries due to their higher hydrogen capacity. Although some improvements have been achieved on the properties of this series of alloys, further research such as the hydrogen sorption properties and electrochemical performance is still needed. This work aims to enhance the hydrogen storage capacity of Ti-V-based BCC phase alloys in a gas-solid reaction and improve their electrochemical characteristics mainly through substituting partial elements, optimizing composition and modifying surface. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and differential scanning calorimetry-thermogravimetry (DSC-TG) techniques were used to study systematically the relationship of the activation, hydrogen sorption and electrochemical performance of the alloys with their microstructure, thermodynamic, and kinetic characteristics. By means of experimental study some general rules which can effectively improve the hydrogen storage performance and electrochemical properties of the alloys have been obtained. In fact, the activation, hydrogen sorption and electrochemical performances of the alloys have been improved significantly.For TiMni.25Cro.25 alloy systems, the oxide layer formed on the surface was found to be one of the important factors making the activation difficult. Addition of appropriate (V4Fe) alloy or carbon can facilitate activation of the alloys. Addition of (V4Fe) alloy to the alloys results in the increase of hydrogen storage capacity and the decrease of plateau pressure and hysteresis factor. It also increases the slope of the hydrogen absorption-desorption plateau. The slope is closely correlated with the strain energy of interstitial site of the alloy. The less the strain energy is, the flatter is the slope of the plateau. In addition, increase of Cr content is effective in depressing the hysteresis and carbon addition is effective in prolonging the cycle life of the alloys.The results for Ti-(50-;t)Mn-10Cr-;cV (x=0, 8, 12, 16, 20, 24, 28, 32) alloys show that, with the increasing V content, the alloys transfer from single C14 Laves phase into two phases of C14 Laves phase and BCC phase, and then into single BCC phase. Increase of V content in the alloys results in an increasing hydrogen absorption capacity, a flatter plateau of hydrogen absorption-desorption, a decreasing plateau pressure and an increasing residual hydrogen capacities during the hydrogen desorption. It is detected that the phase transformation process of BCC? deformed BCC? deformed FCC ? FCC will take place during the hydrogen absorption. As the activation measurements showed thatthe incubation time of activation is correlated with the properties of oxide layer formed on alloy surface, while the reaction rate of alloys with hydrogen is determined by the nature of alloys. In this paper, it is demonstrated that Ti-Mn-Cr-V quaternary alloys with a single BCC structure possess good kinetic characteristics of hydrogen absorption-desorption. The maximum hydrogen absorption and desorption capacities of the studied alloys are about 4wt% and 2.6wt%, respectively, under 373K, which are extensively higher than those of other reported BCC phase alloys.The results of substituting elements and optimizing composition show that Fe addition leads to the increase of hydrogen absorption-desorption plateau pressure and reduces the cost of the alloy. The plateau pressure can be adjusted in the range of 0.02MPa~ 0.26MPa by changing the Ti content. Increase of Cr content is effective in obtaining a single BCC structure, which causes the increase of hydrogen absorption capacity. It is found that the hydrogen storage properties of the alloys are closely correlated with the content of BCC phase and the lattice parameters of the alloys. For a hydrogen storage alloy to keep its lattice parameters in...